Sunday, April 6, 2014

The Long March of the Living Snow

Ames, Iowa (March 1, 2014)

For the last few months this blog has been roaming all around the world, with posts and stories inspired by mountaintops and valleys in four different continents. It is time to get back home; in the freezer, that is. Kero and I had our fun with the frigid winds and powdery snows but one would have thought that by now, April, winter would have let its grip go off the flat plains of Iowa. Not so, we still had a sprinkle of fresh snow on Friday morning. Not as much as in the photo above, but enough to cover with a layer of frost the dead flattened grass waiting for warmer times. It still go below freezing at night.

Snowflakes on my glove

This made me think about a story I heard on radiolab, about snow and the cleverness of life. It goes like this. Snow is made of ice crystals, like the ones in the photo on the left, borrowed from my instagrams. Ice in turn is made of water, cooled below its freezing point. The conversion of water into snow happens in the clouds which are made by floating tiny droplets of water vapor. You want to make snow? Simple, take some of this water flying in the sky, freeze it below 0ºC and voila, snow-on-demand. Well, nature is not so simple, after all. The tricky part is that, to make ice, water is not enough. You need a nucleation point around which the molecules of ice will start to self-organize in the ordered state of solid ice, from the disordered chaotic anarchy of liquid. Without a nucleation point, in fact, water can be cooled well below zero without freezing: supercooled water, like the one I mentioned a few posts ago in reference to cosmic inflation. Ice that won't freeze without a nucleating agent turns out to be a problem, however, if you want to form snow from the pure water in the clouds.

Many sorts of substances can work out as nucleating agents. The best one is ice itself: add a little crystal of ice to supercooled water or vapor, and the little H2O molecules will stop dancing in the wind, to stick just in the right place, growing the crystal like a beautifully sculptured snowflake puzzle. But what if you don't have ice to start with? Well, there are a number of chemicals that mimic the shape of ice crystals, and act as surrogate ice-nucleating points. One of them is a protein secreted by bacteria that like to live on grass. These bacteria are very common, and are found about everywhere in the world. One of their role is to help forming the frost that covers the grass in cold autumn nights, an ice-jacket that protects it from nocturnal below-freezing temperatures.

Bacteria are tiny bugs, however, and it doesn't take much to blow them away. In fact, bacteria likes to be blown away, to be carried by the wind in search of greener pastures. Some bacteria are so light that they can be blown all the way up to the sky: the jet stream is their preferred global transportation system. And that of course presents a problem: the sky is not very conducive to bacterial life, and at some point any good bacterium will need at some point to find a way back to the ground. Bacteria, however, do not have parachutes: how to get safely back to Earth from the heights of sky?

That's where the ice nucleating protein comes handy. Each bacteria in the sky, when the temperature is cold enough, will start nucleating ice around its shell, ultimately growing a large snowflake, heavy enough to precipitate to the ground, but light enough to slow down the fall. Think of this, next time its snowing: for each snowflake falling to the ground, there is a tiny bacterium finding its way home.